Coal planer arrangement with independently positionable cutters



Jan. 2, 1968 A. LGBBE 3,361,479

COAL PLANER ARRANGEMENT WITH INDEPENDENTLY POSITIONABLE CUTTERS Filed Aug. 9, 1965 IN VE N TOR:

ARM/N LOBBE United States Patent once 3,351,479 Patented Jan. 2, 1968 3,361,479 COAL PLANER ARRANGEMENT WITH INDE- PENDENTLY POSITIONABLE CUTTERS Armin Liibbe, Oberaden, Post Kamen, Germany, assignor to Gewerlrschaft Eisenhutte Westfalia, Wethmar, near Lunen, Westphalia, Germany, a corporation of Germany Filed Aug. 9, 1965, Ser. No. 478,486 Claims priority, application Germany, Aug. 12, 1964, G 41,316 6 Claims. (Cl. 299-34) ABSTRACT OF THE DISCLOSURE Coal mining machine adapted to plane a lateral mine face having cutting heads directed toward the mine face with means for quickly adjusting the depth of cut of each cutting head independently, preferably with a scissors linkage support means for the cutting head.

This invention relates in general to a coal planer, or plow, for extracting mineral from a mine face, and more particularly to a coal planer arrangement having one or more independently positionable cutters which permit selective adjustment of their respective depths of extractive engagement with the mine face.

In general, the coal planer arrangement of the instant invention comprises a carriage means disposed for movement relative to a mine face along a predetermined path located in front thereof, at least one, and preferably a plurality of cutter positioning means operatively connected to the carriage means for movement therewith, and an individual motive means also operatively connected to the carriage means for movement therewith, and operatively connected to a corresponding cutter positioning means to selectively position the cutter thereof relative to the carriage means along a direction transverse to the path of carriage means movement to thereby effect a corresponding depth of extractive engagement of said cutter with the mine face.

Each cutter positioning means includes a cutter supported thereby which is disposed for extractive engagement with the mine face to remove mineral therefrom along an extraction path corresponding to the path of carriage means movement, and ordinarily parallel thereto. By arranging the cutter positioning means in a vertically superposed relation to one another, and for independent cutter movement relative to each other along mutually parallel directions by their respective motive means, the individual cutters of such cutter positioning means can be positioned for various selected depths of extractive engagement so as to extract mineral from the mine face along paths disposed in vertical superposition corresponding to that of the cutter positioning means.

-It is already known that a coal planer can be built up upon a base body which supports a number of individual cutter elements arranged one above the other, with the particular number of such cutter elements provided depending upon the thickness of the seam to be mined. In this way, a coal planer can be adapted for mineral extraction from seams of various thicknesses.

On the other hand, it is also known that the mining capacity as well as the power consumed by a plow type coal planer depends substantially upon the profile which the plow cuts into the coal face. Mining capacity and power consumption depend also upon other considerations, such as for example, the resistance presented to the plow cutters by the coal and other minerals which comprise the mine face. Only in the rarest of cases is the coal distributed uniformly over the entire thickness of the mine seam. More frequently, the coal in a mine seam is permeated by packs or strata of rock having various thicknesses and which also may be of very difl erent structural development so as to require a much greater force for removal than would be required if such rock strata were coal instead. In addition, the composition of a mining seam is not ordinarily constant with progressive depth, and it is entirely possible that as the mining along the face thereof proceeds, the composition of the seam will change in such a manner that a different profiling arrangement of the plow cutters to accommodate the composition of the exposed mine face would give a more favorable result with regard to mining capacity and/or plow power consumption.

The coal planer of the present invention is designed with the objective of providing a plow type coal planer having a mine face cutting profile which can be changed rapidly and conveniently.

In the coal planer arrangement of the invention, cutting profile changing can be accomplished speedily enough so that it becomes feasible to determine empirically the optimum profile for the coal face to be mined.

The improved performance afforded by the coal planer of the instant invention is attributable to the fact that the individual cutter positioning devices are supported in super-position above the carriage means, which acts as a movable base support, and are individually adjustable with respect to their depths of cut. Such a coal planer can be expediently constructed with a plurality of base plate members which are stacked at suitable distances apart, one above the other, and which serve for establishing the effective elevations of the corresponding cutter positioning devices in which they are included. These base plates serve as the primary support for the other elements of the cutter positioning devices, and are operatively connected to the carriage means for movement therewith.

A variety of mechanisms can be used for the inde pendently movable cutter positioning devices. For example, a worm or screw driven cross slide, similar to that used in a lathe, can be mounted to each base plate for positioning an associated planing cutter into a selected depth of extractive engagement with the mine face. Alternatively, each cutter can be supported at the free end of a bar or rod, slidably mounted to a base plate so as to permit the cutter to be pushed in deeper or withdrawn for shallower extractive engagement with the mine face. In the preferred embodiment of the invention, a scissor linkage means supported by the base plate is used for positioning each cutter into a selected depth of extractive engagement with the mine face. Thus, the term cutter positioning means is used herein in its broadest sense, to signify any device which is capable of moving a cutter into a selected depth of extractive engagement with the mine face, and which, for such purpose, is adapted to be mounted to the carriage means for movement therewith, and when so mounted is capable of positioning the cutter which it supports relative to the carriage means along a direction transverse, and preferably perpendicular to the direction of carriage means movement.

In accordance with the invention, the positioning movement of the cutter positioning means are effected by corresponding motive means, each of which is preferably a hydraulic actuator, such as the piston and cylinder type, and which can expediently include either hydraulic or mechanical locking means for maintaining the cutter in an established extractive engagement position.

The coal planer arrangement of the invention is susceptible of a variety of embodiments, such as for example, one embodiment including a plurality of cutter positioning means disposed in vertical superposed relation to one another for independent cutter movement relative to each other and to the carriage means along mutually parallel directions by their respective motor means. This particular arrangement permits the individual cutters of the cutter positioning means to be selectively positioned at various corresponding depths of extractive engagement with the mine (face for mineral extraction therefrom along corresponding vertically superposed extraction paths.

Such a coal planer arrangement can be further modified by disposing the uppermost cutter positioning means for cutter movement in a direction inclined in elevation with respect to the directions of cutter movement of the remaining underlying cutter positioning means such that the cutter borne by the uppermost cutter positioning means can be positioned for extractive engagement with the mine face along an extraction path having a selected depth and a selected elevation above the extraction paths of the cutters of the underlying cutter positioning means. This enables the uppermost cutter to follow the uppermost boundary of the mine seam to cut the mine roof therealong. This permits the recovery of overhanging top coal, which in conventional coal planets of the plow type, is customarily recovered by means of special roof cutters, or with sets of loosening tools arranged above the other cutters carried by the plow.

In accordance with the invention, this function can be performed by the uppermost cutter positioning means, which for such purpose is mounted so as to be swingable in elevation relative to the carriage means and the other cutter positioning means about an axis parallel to the carriage means movement path, i.e. ordinarily parallel to the mine face. Such elevation swinging can be expediently accomplished by means of a hydraulic actuator which is operatively connected to the carriage means and to the uppermost cutter positioning means to position same into a selected position of inclination for roof cutting operations. In this connection, either hydraulic or mechanical locking means are preferably provided for maintaining the uppermost cutter positioning means in the position of inclination established by the hydraulic actuator.

For this uppermost cutter positioning member, the provision of an elevation swing capability results in an additional degree of freedom in cutter depth control, in that the same cutter movement capability as provided in the underlying cutter positioning means is also provided [for the uppermost, and hence the cutter of the uppermost cutter positioning means has an angular positioning capability about the elevation swing axis, as well as a radial positioning capability relative thereto.

According to a preferred embodiment of the invention, each cutter positioning means includes a base plate operatively connected to the carriage means for movement therewith, a scissor linkage means supported by the base member with the cutter being operatively connected to the scissor linkage for support thereby. The scissor linkage is operatively connected to a hydraulic actuator motive means for cutter positioning movement thereby.

For simplicity, the scissor linkage means is preferably constructed with a single pair of link members articulately connected to each other, with the cutter being connected to the linkage for movement with the articulate connection point of the two link members. The hydraulic actuator is connected at each of its ends to a corresponding link member, and effects cutter positioning movement by varying the relative angular spacing between the link members, thereby causing their articulate connection point to travel in toward the carriage means, or outwardly therefrom along a direction which is preferably perpendicular to the path of carriage means movement.

Of course, scissor linkages having more than one pair of link members can be used, in which case the cutter is mounted for movement with the articulate joint furthest away from the carn'age means. Apart from the advantages of obtaining a multiplication of cutter movement over that which could be obtained with a single pair of link members, it is questionable as to whether a scissor linkage having repeated pairs of link members cross-connected in tandem would provide any practical advantage over the single link pair type.

One of the advantages in using scissor linkages for positioning the cutters lies in the fact that the hydraulic actuator used for controlling their spread angle can be arranged for extension and contraction along paths'which lie approximately parallel to the path of carriage means movement, since by the inherent nature of a scissor linkage, movement of the articulating cross connection joints occurs in a direction approximately perpendicular to that in which the link legs are spread. Thus, through the use of a scissor linkage for cutter positioning, a considerable saving in working space behind the carriage means is provided, because if the cutters are mounted at the free ends of individual hydraulic rams, a space corresponding 7 in width to the length of such rams would be required.

The displacement of the scissor linkages or other mechanisms used for supporting the cutters can be effected by other types of motive means, such as for example, motor driven turnbuckles, and by pneumatic as well as by hydraulic cylinder actuators. In the case of either hydraulic or pneumatic actuators, individually provided for each cutter positioning device, each actuator can be provided with an associated accumulator operatively connected therewith for supplying pressurized working fluid thereto. Particularly in the case of hydraulic actuators, the provision of such accumulators will enable the actuators to urge their respective cutters via the scissor linkages into extractive engagement with the mine face with a force resiliently limited by the accumulator. 7

However, if desired a common pressurized fluid container or accumulator can be provided in lieu of such individual accumulators. In such case, the hydraulic accumulator is operatively connected to a hydraulic fluid distribution means, such as for example a control valve, which is in turn operatively connected to each hydraulic actuator for suppl g pressurized hydraulic fluid to a selected combination of said actuators for cutter positioning movement thereby.

By means of a hydraulic or pneumatically controlled operation of the individual cutter positioning means, the cutters acting below a pack of rock on the mine face can, for example, be moved forward to a greater or lesser extent with respect to cutters acting at the height of the rock pack. By these and similar measures, the results can be obtained that the packed rock will ordinarily drop off by itself such as not to require any cutting action by the cutters. This is particularly advantageous in that the cutters acting on rock strata 0r packs are subjected to particularly high wear, and such wear can be avoided easily by the coal planer arrangement according to the invention.

The cushioning efiect provided by one or more hydraulic accumulators flow connected with the actuators will enable the individual cutters to be moved back from the mine face automatically Whenever they encounter an increased cutting resistance. Furthermore, a hydraulic adjustment among the several cutters can be provided in the form of a closed hydraulic system having communicating action with the respective actuators. In such case, the same cutting resistance response would be automatically established among all the cutters.

It is therefore an object of the invention to provide a coal planer arrangement for extracting mineral from a mine face by means of planing cutters which can be independently positioned relative to one another to provide a selected extraction depth profile.

Another object of the invention is to provide a coal planer arrangement as aforesaid wherein the cutters are arranged in superposition for extractive engagement along correspondingly elevated paths on the mine face. A further object of the invention is to provide a coal planer arrangement as aforesaid wherein the uppermost cutter can be elevated into a selected position of extractive engagement with the mine face to cut a mine roof surface which follows the upper boundaries of themine seam.

A further object of the invention is to provide a coal planer arrangement as aforesaid which can be operated either hydraulically or pneumatically.

A further object of the invention is to provide a coal planer arrangement as aforesaid wherein the cutters are resiliently urged into extractive engagement with the mine face to accommodate variations in cutting resistance without excessive wear or breakage.

A further object of the invention is to provide a coal planer arrangement as aforesaid wherein the cutters can be adjusted to avoid cutting into rock strata.

Still another and further object of the invention is to provide a coal planer arrangement as aforesaid which is relatively compact with regard to operating clearance space required.

Other and further objects and advantages of the invention will become apparent from the following detailed description and accompanying drawing in which:

FIG. 1 is a schematic plan view of a coal planer arrangement according to a preferred embodiment of the invention.

FIG. 2 is an end elevation view of the coal planer of FIG. 1.

Referring now to FIGS. 1 and 2, the coal planer arrangement A is provided with a carriage means C disposed for movement relative to the mine face F along a predetermined path located in front thereof and established by the guide track T resting upon the mine floor G. The carriage means is expediently constructed with a pair of carriages and 11 which are longitudinally spaced apart from each other on the track T, and which respectively support bottom cutter holders 12 and 13. The carriages 10 and 11 are joined for movement in unison by a bridging superstructure 14, said superstructure 14 also serving as a means for operatively connecting the cutter positioners 15, 16, 17, 18 and 19 to the carriage means C for movement therewith.

Depending upon the thickness of the coal seams which will be mined, the number of cutter positioners -19 can be varied, and even though the uppermost cutter positioner 19 can be swung upward for mine roof cutting action, as indicated by phantom outline in FIG. 2, a plurality of cutter positioners 15-19 is preferable to a single one.

The bottom cutters mounted on the holders 12 and 13 are so arranged that when moved back and forth by the carriage means C along the mine face F in extractive engagement therewith, they extract mineral therefrom in both directions of movement for a more eii'icient bottom extraction operation.

The carriages 10 and 11 can be expediently of similar gantry-like construction, such as is common in the long Wall mining industry, and are adapted for movement in guided engagement with the track T.

Each of the cutter positioning means 15-19 includes a cutter 23 supported thereby and disposed for extractive engagement with the mine face F to remove mineral therefrom along an extraction path corresponding to and ordinarily parallel to the path of movement of the carriage means C.

To effect independent cutter positioning movement, each of the cutter positioners 15-19 are operatively connected to a corresponding motive means represented by the individual hydraulic actuators P, each of said bydraulic actuators P being of conventional construction and having a cylinder member 24 and a piston member 25 disposed for relative sliding movement. By controlling the flow of hydraulic fiuid from an accumulator 26 via a distribution valve 27 to the several actuators P, their corresponding cutter positioners 15-19 can be driven either singly or in a selected combination to position their respective cutters at selected depths of extractive engagement with the mine face F.

As will be apparent from the view of the uppermost cutter positioner 19 shown in FIG. 1, the linear relative movement of the piston 25 and cylinder 24 in each actu- 6 ator P takes place along a direction approximately parallel to that of carriage means C movement, and said actuator movement is translated by the link members 21 and 22 into a cutter 23 movement which takes along a direction transverse and approximately perpendicular to the carriage means C movement.

From FIG. 2 taken together with FIG. 1, it can be seen that the several cutter positioners 15-19 are as to their basic construction, substantially similar and are arranged in vertical superposed relation, one above the other such that the extraction paths cut by their respective cutters 23 are substantially parallel and disposed in a corresponding superposed elevation relationship.

The uppermost cutter positioner 19 is swingable in elevation relative to the carriage means C about the axis of the pivot pin 29, which is ordinarily horizontal and parallel to both the mine face F and path of cutter means C movement. Preferably two pivot pins 29 arranged coaxially at each end of the cutter positioner 19 are provided for effecting the pivot connection to the superstructure 14 which permits such elevation swinging of the cutter positioner 19. This elevation swinging can be readily effected by a hydraulic actuator 30 which is pivotally connected at its opposite end to the superstructure 14 and to the cutter positioner 19. By providing an elevation swing capability for the upper most cutter positioner 19, it becomes possible with a coal planer A of limited height and fewer stacked cutter positioners 15-19 to cut the roof surface (not shown) at the upper boundary of a coal seam which is appreciably thicker than the height of the coal planer A with its cutter positioners 15-19 stacked in a horizontal configuration.

In FIG. 2, the several individual cutter positioners 15- 19 are arranged above the bottom cutter holders 12 and 13 in a typical operating configuration to illustrate that a substantial difference between their respective cutting depths can be achieved by the independently movable cutter positioners 15-19 in the coal planer A of the instant invention. However, it is to be understood that the actual profile cut into the coal face F can be varied by appropriately positioning the individual cutters 23 via their respective cutter positioners 15-19, with the particular profile shown in FIG. 2 being intended merely as an illustrative example.

While a variety of cutter positioning mechanisms can be used to accomplish the same independent cutter positioning movement contemplated by the invention, the cutter positioners 15-19 shown in FIGS. 1 and 2 represent a preferred cutter positioning means.

Essentially the cutter positioners 15-19 each include a base plate member 20 which is operatively connected to the carriage means C for movement therewith via the superstructure 14, and a scissor linkage means S supported by said base plate 29. In each cutter positioner 15-19, the cutter 23 is operatively connected to the scissor linkage means S for positioning movement thereby in a direction approximately perpendicular to the path of carriage means C movement.

The base plates 20 are so constructed and arranged that they can be stacked one one top of the other at suitable spaced-apart distances, and secured to the superstructure 14 to provide a substantially rigid support for the movable scissor linkages S, and also to strengthen the bridging connection of the superstructure 14 to the carrrages 10 and 11.

As can be appreciated by the artisan, the particular detalls of the base plates 20, superstructure 14 and the connection of the base plates 20 therewith can be varied as desired.

While the cutter positioners 15-19 can be constructed with scissor linkages (not shown) having multiple pairs of tandem, cross-connected link members, operating substantially upon the same mechanical principle as the scissor l nkages S shown having a single pair of link members 21 and 22, apart from the increased cutter 23 displacement amplitude for a given actuator P movement with a multi- 23 shank portion 32 and serves as a support connection between said cutter 23 and the scissor linkage S such that the cutter 23 is movable with the pivot pin 31, i.e. the articulate connection joint of the link members 21 and 22. To prevent the cutter 23 from rotating about the pin 31, an additional pair of stabilizing links 33 and 34 are provided. The links 33 and 34 are pivotally connected at one end to each other and to the cutter shank 32 by means of a common pivot pin 35. The opposite ends of the links 33 and 34 are connected by pivot pins 36 and 37 to the link members 22 and 21 respectively so as to form a parallelogram linkage type of support for the cutter 23, whereby said cutter 23 is maintained in the same orientation relative to the links 21 and 22, and the carriage means C regardless of its lateral position relative thereto.

Of course, it is to be understood that other cutter 23 stabilizing means can be substituted for the parallelogram linkage shown. For example, a rigid bar or rod (not shown) could be connected to the end of the cutter shank 32 and passed through a guide slider (not shown) mounted to the base plate 21 to accomplish the same result. In such case, the cutter 23 would be free to move with the scissor linkage S, but would be prevented from rotating by the slidably guided rod (not shown).

The hydraulic actuator P is operatively connected to the ends of the link members 21 and 22 opposite to their articulate connection ends, so that the angle of separation between said link members 21 and 22 can be varied in accordance with the effective length of the actuator P, thereby causing the cutter 23 to travel in and out with respect to the carriage means C for corresponding depths of extractive engagement with the mine face F.

For this purpose, the cylinder 24 and piston 25 of the actuator P are pivotally connected by means of pins 38 and 39 to the link members 21 and 22 respectively. The pins 38 and 39 extend into guide slots 40 and 41 respectively provided in the base plate 20 for permitting limited relative angular movement of the links 21 and 22 as necessary for positioning the cutter 23. To prevent the links 21 and 22 from lifting up away from the base plate 20, the

pins 38 and 39 can be of the type having a shoulder or head for engaging against the underside of the base plate 20 in the slots 40 and 41. In this way, the links 21 and 22 can be more effectively supported by the base plate 20, and yet be free to execute their cutter 23 positioning movements.

One of the advantages of the scissor linkage S used in the cutter positioners 15-19 lies in its inherent feature that the actuator P can be arranged for movement in a direction approximately perpendicular to that of the cutter 23 positioning, i.e. the actuator P can be positioned for movement in the same direction as the carriage means C movement, thereby resulting in a saving of clearance space behind the carriage means C, as compared with the clearance space required if the cutter 23 were mounted for movement in the same direction by a hydraulic ram (not shown). 7

By regulating the pressure of the hydraulic fluid supplied to the actuators P, the bearing force of the cutters 23 against the mine face F can be controlled.

The pressurized hydraulic fluid for actuating the several actuators P is preferably stored for supply thereto in a hydraulic accumulator 26. In this way, the cutters 23 supported by the corresponding scissor linkages S will be urged into extractive engagement with the mine face F with a force resiliently limited by the cushioning effect of the accumulator 26.

With regard to the hydraulic system arrangement for controlling the cutter positioners 15-19, either an individual accumulator 26 can be used for each actuator P, with the hydraulic fluid flow thereto being controlled via a separate distributing valve 27, or a single common accumulator 26 can be used for supplying all of the actuators P via a multiple outlet distributing valve 27, to effect the same result.

Regardless of the particular hydraulic system arrangement chosen, in accordance with the invention, the valve or valves 27 are operatively connected between the common hydraulic accumulator 26 or individual accumulators 26 and the actuators P for energizing same either singly or in any selected combination for corresponding cutter 23 positioning action.

As can be appreciated by the artisan, the detailed arrangement of the hydraulic control for the several actuators P can be conventional control arrangements or obvious modifications thereof which will accomplish the aforesaid objectives. Likewise, either single acting or double acting hydraulic actuators P can be used, as well as pneumatic actuators P.

The pressurized hydraulic fluid for the actuation of the several hydraulic actuators P is thus stored in the hydraulic accumulator 26 and is so controlled via the fluid dis tribution valve 27 that the cutters 23 of one or more of the cutter positioners 15-19 are advanced or pulled back in the desired manner.

It is understood, however, that a suitable primary source of pressurized hydraulic fluid is provided for connection to the foregoing hydraulic actuators P and accumulator 26, in order that a continuous cutter positioning capability will exist. For example, a collector 28 can be expediently provided to collect spent hydraulic fluid discharged from the various actuators P for recirculation by a pump (not shown) in the primary source of hydraulic fluid.

By providing communicating hydraulicflow action between the several actuators P and a common accumulator 26, the individual cutters 23 can be made automatically self-adjusting to a selected cutting resistance.

In positioning the cutters 23 for selected cutting depths, locking means, such as mechanical (not shown) or hydraulic, as for example, by means of valves 27 which seal off the flow of hydraulic fluid into and out of the actuators P, are preferably provided to assure that in each case, the intended cutting depth will be maintained. Likewise, similar locking means can be provided for maintaining the uppermost cutter positioner 19 in a selected attitude of inclination. By reason of the substantial incompressibility of liquids, such position locking can be simply accomplished where hydraulic actuators P and 35 are used. For example, by sealing off fluid flow from the actuators P, further relative movement of the cylinder 24 and piston 25 can be prevented. If it is desired to provide. a resilient type of locking action, an air chamber type of hydraulic accumulator 26 can be provided together with means (not shown) for increasing the air pressure therein to controllably vary'the resilient characteristics thereof.

What is claimed:

1. Coal planer comprising carriage means adapted to move along a predetermined path in front of a mine face;

a multiplicity of cutter positioning means, each of which cutter positioning means has a cutter mounted thereon for engagement with said mine face; which cutter positioning means includes a scissor linkage means supportmg said cutter and operatively connected to a motive means adapted to position the cutter in a direction transverse to the direction of motion of the coal planer at a selective depth with respect to said coal face.

2. The coal planer arrangement according to claim 1 wherein each corresponding motive means includes a hydraulic actuator operatively connected to said scissor linkage means for positioning the cutter supported thereby.

3. A coal planar arrangement, which comprises a carriage means disposed for movement relative to a mine face along a predetermined path located in front thereof, at least one cutter positioning means operatively connected to said carriage means for movement therewith, each cutter positioning means including a base member operatively connected to Said carriage means for movement therewith, a scissors linkage including a pair of linked members articulately connected to each other, Which scissors linkage is operatively connected to a cutter and supports said cutter for movement thereof with the articulate connection point of said scissors linkage, and is operatively connected to motive means for cutter positioning movement in a direction transverse to the movement path of said carriage means, which motive means includes a hydraulic actuator operatively connected to said scissors linkage for selectively varying the angle of separation of the members thereof, Whereby to correspondingly position their articulate connection point and thus position said cutter.

4. The coal planer arrangement according to claim 3 wherein each corresponding motive means also includes a hydraulic accumulator operatively connected to the hydraulic actuator thereof for supplying pressurized hydraulic fluid thereto for urging the cutter supported via the corresponding scissor linkage into extractive engagement with the mine face with a force resiliently limited by said accumulator.

5. The coal planer arrangement accordng to claim 3 including a hydraulic accumulator and hydraulic distribution means operatively connected to said accumulator and to each hydraulic actuator for supplying pressurized hydraulic fluid to a selected combination of said actuators for urging the cutters supported by corresponding scissor linkages into extractive engagement with the mine face with respective forces resiiiently limited by said accumulator.

6. The coal planer arrangement according to claim 5 wherein said hydraulic actuators are in floW communication With the hydraulic accumulator to resiliently limit the forces exerted by said cutters to a common value.

References Cited UNITED STATES PATENTS 2,666,629 1/1954 Sproul 29932 FOREIGN PATENTS 148,726 4/1951 Australia. 935,012 8/1963 Great Britain.

ERNEST R. PURSER, Primary Examiner. 

